Fuel cells are emerging as practical and versatile power sources, which can be more efficient and less environmentally damaging than rival technologies. From mobile phones and electrical vehicles to spacecraft and multi-megawatt power stations, the application potential for fuel cells is growing rapidly. Fuel cells have much in common with batteries, which also convert energy that is stored in chemical form into electricity. In contrast to batteries, however, they oxidize externally supplied fuel and therefore do not have to be recharged.
Fuel cell operated electric vehicles are today visualized as the way to meet growing human mobility requirements worldwide, replacing fossil fuels in the distant future and assuring a more environmental-sustainable approach to mobility.
Fuel cells can be configured in numerous ways with a variety of electrolytes, fuels and operating temperatures. For example, fuels such as hydrogen or methanol can be provided directly to the fuel cell electrode or fuels such as methane or methanol can be converted to a hydrogen rich gas mixture external to the cell itself (fuel reforming) and subsequently provided to the fuel cell. Air is the source of oxygen in most fuel cells, although in some applications, the oxygen is obtained by hydrogen peroxide decomposition or from a cryogenic storage system.
Although there are theoretically a limitless number of combinations of electrolyte, fuel, oxidant, temperatures and so on, practical systems are in many cases based on polymer electrolyte membrane fuel cell (PEMFC) technology, characterized by solid polymer electrolyte systems using hydrogen as the fuel source and oxygen or air as the oxidant. Further, the PEMFC can be miniaturized as compared with other types of fuel cells and is suitable as mobile power source or as small capacity power source.
The polymer electrolyte membrane forming the heart of the PEMFC acts as a proton-exchange membrane, and must have excellent ion conductivity, physical strength, gas barrier properties, chemical stability, electrochemical stability and thermal stability at the operating conditions of the fuel cell.
Actually, the electrode reactions taking place in a hydrogen PEMFC are:
Anode Reaction:H2→2H++2e−Cathode Reaction:½O2+2H++2e−→H2OYielding as Overall Electrochemical Reaction:H2+½O2→H2O.
As one of the critical features of a PEMFC is to maintain a high water-content in the membrane to assure acceptable ion conductivity, water management in the membrane is critical for efficient performances; the PEMFC must operate in conditions wherein the by-product water does not evaporate faster than it is produced. The water content of a PEMFC is determined by the balance of water or its transport during the reactive mode of operation. Water-transport processes are a function, notably, of the current and the properties of both the membrane and the electrodes (permeability, thickness, etc.). Influencing the water transport are the water drag through the cell, back diffusion from the cathode (most of the product water is traditionally removed from the cathode, where it is produced, by excess flow of oxidant gas), and the diffusion of any water (either by-product or supplied as moisture in reactants) in the membranes.
Most of the car manufacturers are today driving the development of fuel cell cars based on the use of dry reactants (air and hydrogen) and high operating temperatures aiming at fuel cell system (stack and auxiliaries) simplification and reduction of steric hindrance of the same.
Nevertheless, among PEMFC components, the ionomeric membrane is the weak component most sensitive against operations simultaneously at high temperature and with dry reactants.
Membranes currently used in PEMFC are perfluorinated sulfonic acid (PFSA) polymers such as NAFION® resins from DuPont. Even if such membranes have demonstrated good performances, appreciable long-term stability in both oxidative and reductive environments and valuable protonic conductivity under fully hydrated conditions (80-100% relative humidity, RH hereafter), they are limited to low temperatures (up to 80° C.), require a sophisticated water management (system complexity), and due to limited stability in limited hydration and high temperature conditions, are unpractical from a durability point of view.
For instance, EP 1589062 (SOLVAY SOLEXIS S.P.A.) 26/10/2005, discloses ionomeric membranes comprising (per)fluorinated ionomers suitable for being in fuel cells under fully hydrated conditions.
Great efforts have been made in academic and industrial laboratories to deliver proton exchange membranes for hydrogen-based fuel cells able to operate in so-called “dry conditions”, i.e. without the need of sophisticated water management systems, and/or at temperatures up to 120° C.
Within this scenario, U.S. Pat. No. 7,094,851 (GORE ENTERPRISE HOLDINGS, INC.) 22 Aug. 2006 discloses ionomers having low equivalent weight (typically between 625 and 850 g/eq), high conductivity (greater than 0.13 S/cm) capable of being processed into thin film and are extremely well-suited for low humidity or high temperature fuel cell applications. Nevertheless, ionomers hereby described comprising recurring units derived from tetrafluoroethylene (TFE) and from a comonomer of formula (A):
wherein X is F, Cl or Br or mixtures thereof; n is an integer equal to one or two; Rf and R′f are independently selected from the group of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkyl radical; Y is an acid group or a functional group convertible to an acid group, like notably —SO3Z, with Z being H or any combination of cations; a is zero or an integer greater than zero; and b is an integer greater than zero, are known to possess poor temperature resistance, so that membranes prepared therefrom cannot withstand long-life PEMFC operations at temperatures exceeding 65° C.
Similarly, U.S. Pat. No. 7,041,409 (GORE ENTERPRISE HOLDINGS, INC.) 9 May 2006 discloses fluorinated ionomeric co-polymers comprising:
(a) a substantially fluorinated backbone;
(b) pendant groups derived from an ionomeric monomer of the formula (A)
wherein X is F, Cl or Br or mixtures thereof; n is an integer from zero to two; Rf and R′f are independently selected from the group of F, Cl, perfluoroalkyl radical, and chloroperfluoroalkyl radical; Y is an acid group or a functional group convertible to an acid group, like notably —SO3Z, with Z being H or any combination of cations; a is zero or an integer greater than zero; and b is an integer greater than zero; and(c) pendant groups derived from a vinyl ether monomer that has at least two vinyl ether groups of the form, CA2=CB—O—, where the vinyl groups are separated by greater than four atoms; A is independently selected from the group containing F, Cl, and H; and B is independently selected from F, Cl, H and ORi, where Ri is a branched or straight chain alkane that may be partially, substantially or completely fluorinated or chlorinated,said copolymers being particularly well-suited for low humidity of high temperature fuel cell operations. Nevertheless, such ionomers exhibit limited durability in high temperature PEMFC operations, i.e. at temperatures exceeding 65° C.
There is still a need in the art for a PEMFC comprising a fluoroionomer membrane able to sustain operations in dry conditions (i.e. operating with reactants having low dew point) while maintaining outstanding energetic performances (efficiency) and durability.